2003 Buick Rendezvous warning chime

Once in a while while driving on a trip, the warning chime will sound once. No warning lights come on. I have checked fluids, air pressure, gas cap being tight and find nothing out of order. This seems to happen at random and not often. When it happens, it usually only does it once that trip although one time it did it twice in the same day. It never happens on short drives.